The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer and a FAX (facsimile telegraph) wherein an image forming body is surrounded by a charging means, an image writing means and a developing means, and a toner image formed on the image forming means is transferred onto and fixed on a transfer material.
In two-sided image forming, there has so far been employed a method wherein an image on one side of a document formed on an image forming body is transferred and fixed on a transfer material which is then kept temporarily in an intermediate tray, and the transfer material is fed out of the intermediate tray in synchronization with an image formed next on the image forming body so that the image is transferred and fixed on the other side of the transfer material.
In the two-sided image forming apparatus of this type, conveyance operations for a transfer material such as conveyance thereof to an intermediate tray and conveying the transfer material through a fixing unit two times are conducted as stated above. Therefore, reliability for conveyance of the transfer material is low, resulting in causes for jamming and creases on the transfer material. For the situation mentioned above, TOKKOSHO Nos. 49-37538 and 54-28740 and TOKKAIHEI Nos. 1-44457 and 4-214576 are suggesting an image forming apparatus wherein toner images are formed on both sides of a transfer material and then they are fixed at the same time.
Further, the inventors of the invention are studying two-sided image forming by making and using an image forming apparatus wherein there is arranged a toner image forming means composed of a charging means, an image writing means and a developing means arranged around an image forming body (photoreceptor drum), and toner images formed on the image forming body are temporarily transferred onto a belt-shaped intermediate transfer body (a toner image receiving body) by the first transfer means, then, toner images are formed on the image forming body again, and the first transfer means transfers the toner images on the image forming body on one side of a transfer material conveyed on the intermediate transfer body as an obverse side image, while the second transfer means transfers the toner images on the intermediate transfer body on the other side of the transfer material as a reverse side image, and after that, the transfer material is neutralized by a transfer material separating means to be separated from the intermediate transfer body, thus toner images on the transfer material are fixed for two-sided image forming.
However, when an intermediate transfer body having low volume resistivity is used in the image forming apparatus mentioned above, the intermediate transfer body is struck by arcing of the second transfer means or the transfer material separating means before and behind or at both sides of the transfer material, and the intermediate transfer body is damaged, resulting in image defect of the reverse side image and transfer trouble of the obverse side image, which are the problems. On the other hand, when the intermediate transfer body having the high volume resistivity is used, electric charges on the intermediate transfer body charged by the second transfer means do not leak before arriving at the separating position of the transfer material, and thereby the transfer material after being subjected to transferring by the second transfer means is electrostatically attracted to the intermediate transfer body firmly, which causes a problem of difficult separation. For that reason, a transfer material separating means is provided at the position where a transfer material is separated from an intermediate transfer body, but when the second transfer means and the transfer material separating means are intended to be provided to face a roller that is located at the separation position to spread the intermediate transfer body, it is necessary to make the second transfer means and the transfer material separating means to be small in size. Therefore, electric discharge of both the second transfer means and the transfer material separating means is made insufficient, or the second transfer means and the transfer material separating means need to be arranged closely each other, which causes problems that electric discharge of the second transfer means and that of the transfer material separating means interfere, or transfer of toner images from the intermediate transfer body to the reverse side of the transfer material and separation of the transfer material from the intermediate transfer body are not carried out stably.
In addition, when the second transfer means is arranged in the vicinity of the separation position, transfer of the reverse side image conducted by the second transfer means is affected by how the transfer material is separated from the intermediate transfer body, regardless of whether the transfer material separating means is present or not, which causes problems that transfer of toner images from the intermediate transfer body to the reverse side of the transfer material can not be carried out stably, and in particular, the reverse side image is disturbed or transfer trouble happens at the trailing edge of the transfer material.